The present invention relates to a method for transmitting high-definition TV (HDTV) signals compatible with the standards currently in use (NTSC,
or SECAM), i.e. such as to be receivable and usable by conventional TV sets, even if with normal picture quality. The invention also relates to a process for receiving and using said signals with HDTV quality.
As is known, HDTV systems have been proposed in which the picture is scanned in a number of lines which is approximately twice that of the current standard, e.g. 1250 lines instead of 625 lines per frame according to the European standards, or 1125 lines instead of 525 according to the U.S. standard. Though the high TV definition obtainable with such systems is certainly desirable, their adoption entails two severe disadvantages, i.e. the high frequency bandwidth they occupy and the non-compatibility of the signal with current TV receivers.
To eliminate the first of these disadvantages, picture compression methods, for example the MUSE transmission system developed by the Japanese public television company Nippon Hoso Kyokai, have been proposed. According to this system, only the fixed portions of the picture (i.e. the picture portions which remain unchanged from one frame to the next, such as the background) are transmitted in high definition, while the moving portions of the picture, for which the human eye perceives a lower definition, are transmitted with reduced definition. Motion vectors, indicating the movements of the entire picture, are also transmitted together with the signal (see Hayaski K., "Research and Development on High-Definition Television in Japan", SMPTE Journal, March 1981, or NHK--Technical Monograph No. 3: "High-Definition Television", Nippon Hoso Kyokai--Technical Research Laboratories, June 1982).
HDTV, even in its various versions, such as the abovementioned MUSE system, still has the disadvantage of non-compatibility, i.e. of the impossibility of receiving HDTV programs with an ordinary TV receiver, not even with a low-quality picture, due to the difference in transmission standards.
Besides the difference in the number of scanning lines, it should be noted that the aspect ratio of the picture is also different in the HDTV system, being set to 16/9 in the new system as opposed to 4/3 in conventional standards (NTSC, PAL or SECAM).